Is loop quantum gravity wrong? - physicist explains | Don Lincoln and Lex Fridman
Don Lincoln explains the differences between loop quantum gravity and string theory, noting that loop quantum gravity attempts to quantize space itself rather than unify all forces. He discusses how an early prediction of loop quantum gravity — that light speed would vary by frequency — was disproven by gamma ray burst observations, but the theory adapted. He also highlights the landmark gravitational wave observation confirming that gravity travels at the speed of light.
Summary
Don Lincoln begins by distinguishing loop quantum gravity from other speculative theories, noting it is better developed and makes actual predictions. Crucially, he clarifies that loop quantum gravity is not a theory of everything — it solely attempts to quantize gravity, treating space as having a smallest indivisible unit, analogous to how water is ultimately composed of discrete molecules. This contrasts with Einstein's general relativity, where space is treated as smooth, continuous, and infinitely divisible.
Lincoln then explains the origins of string theory, which was originally developed as a theory of the strong nuclear force and competed directly with QCD (quantum chromodynamics). String theory ultimately lost that competition, but gained new life when researchers noticed it predicted a zero-mass, spin-2 particle — which can be proven to be the graviton. This unexpected feature transformed string theory into a candidate theory of everything, generating enormous excitement in the theoretical physics community.
Returning to loop quantum gravity, Lincoln recounts how an early version of the theory predicted that the speed of light would vary depending on frequency — higher frequency light traveling at a different speed than lower frequency light, due to interaction with the discrete structure of space. Observations of gamma ray bursters, which emit light across all wavelengths from billions of light years away, showed no such disparity in arrival times, seemingly disproving the theory. However, Carlo Rovelli contacted Lincoln to clarify that loop quantum gravity had already revised and abandoned that prediction, meaning the observation invalidated only the older version of the theory.
Finally, Lincoln discusses the landmark 2017 observation of gravitational waves and light from two merging neutron stars, located 140 million light years away. Both the gravitational waves and the light arrived within 1.7 seconds of each other, providing empirical confirmation that gravity travels at the speed of light — a long-held assumption that was now experimentally proven for the first time.
Key Insights
- Lincoln clarifies that loop quantum gravity is not a theory of everything — it only attempts to quantize gravity and understand the discrete structure of space, unlike string theory which aims to unify all forces.
- Lincoln explains that string theory was originally developed as a theory of the strong force and was losing to QCD, but was rescued when someone noticed it predicted a zero-mass spin-2 particle, which is necessarily the graviton — transforming it into a candidate theory of everything.
- An early version of loop quantum gravity predicted that the speed of light would vary by frequency due to interaction with the granular structure of space, but gamma ray burst observations showed all wavelengths arriving simultaneously, disproving that specific prediction.
- Carlo Rovelli pushed back on Lincoln's claim that loop quantum gravity was 'killed,' explaining the theory had already been revised to drop the variable speed of light prediction, meaning the gamma ray burst data only falsified the old version of the theory.
- The 2017 observation of gravitational waves and light from merging neutron stars 140 million light years away, arriving within 1.7 seconds of each other, provided the first direct empirical confirmation that gravity travels at the speed of light.
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